1. Field of the Invention
The present invention is directed to a gradient coil system for a nuclear magnetic resonance tomography apparatus of the type wherein individual gradient coils are secured to a tubular carrier which surrounds an examination volume.
2. Description of the Prior Art
European Application 0 073 402 discloses a gradient coil system of the type described above. The gradient coils for the x, y and z directions are mounted on a tubular carrier with the gradient coils for the x-direction and for the y-direction being saddle coils.
Pulsed currents having an order of magnitude of 100-200 A and a switching time of less than 1 ms flow in the gradient coils. The mechanical forces resulting from the interaction of the conductors with the magnetic fields excite the gradient coil system and cause the system to vibrate as a unit. This results in a knocking noise which can be discomforting for the patient. The noise generation cannot be reduced to a satisfactory degree with conventional damping measures alone. Attempts have therefore been undertaken to reduce the creation of the noise by winding the coil system on a self-supporting basket-like (woven) arrangement. The sound emission surface and thus the noise generation are thus reduced. Examples of such designs are disclosed, for example, in European Application 0 304 127 and in German OS 34 06 052. The level of acoustical pressure, however, cannot be reduced to a satisfactory degree even with these designs.
Placement of the gradient coils in the vacuum of the cryostat is disclosed in European Application 0 123 075. Another proposal is to separate the examination space from the gradient coils using an evacuated double carrier. Such designs, however, would require high material and construction outlay, and moreover cannot prevent the transmission of acoustical vibrations through the structure itself.
Another proposal is disclosed in German OS 40 17 260 wherein the windings of the gradient coil are cast in an epoxy resin and are combined to form a coil system. This arrangement, however, still does not satisfactorily diminish the generation of noise, because noise having substantial low-frequency components is difficult to muffle.